This case-control epidemiological study will examine the relationship between use of oral contraceptives and fibrocystic breast disease categorized histopathologically according to the extent of epithelial atypia; the specific hypothesis will be tested that there is a positive association between long-term use of oral contraceptives and the degree of epithelial atypia. Cases will be women who have had fibrocystic breast disease diagnosed from biopsy at four large Connecticut hospitals over a 22-month period. Their slides will be read by a pathologist associated with the study, and classified according to extent of epithelial atypia. Controls will comprise women admitted as inpatients or outpatients to certain surgical services of these same hospitals. Information on oral contraceptive use, demographic characteristics, and other variables of interest will be obtained from cases and controls by means of a structured questionnaire administered by trained interviewers, supplemented by queries to physicians. Previous use of oral contraceptives and exposure to other factors of interest will be compared among four groups categorized according to extent of epithelial atypia, as well as between each of the four groups and controls. This study should add to our knowledge of the epidemiology of fibrocystic disease and potentially to our understanding of the epidemiology of breast cancer, since fibrocystic disease with marked epithelial atypia has been found to be the form of fibrocystic disease most strongly associated with subsequent breast cancer.